The present invention relates to tape measures, more particularly the present invention relates to tape measure locking systems.
Carpenter's tape measures are commonly used devices which typically incorporate a spring-loaded rolled metal or plastic tape into a housing. To use the tape, the end of the tape or blade, which protrudes from the housing, is pulled to the desired length and the measurement is taken. The tape measure may include a locking means, such as a toggle lock, for locking the tape at a set length to enable repeat measurements.
After all measurements are taken, the tape is permitted to retract into the housing. By releasing the locking mechanism, the spring motor within the housing acts on the rolled tape to effect its retraction.
There are a number of problems associated with existing tape measures, ranging from poor ergonomics to insufficient operation. These tape measures are often large, bulky devices, heavy and difficult to hold and operate. The locking mechanisms are inadequate because of susceptibility to wear and the expense of manufacture. Moreover, the position of the button for actuating the mechanism is often difficult to reach, detracting from ease of operation.
The weight and size have been, to a large extent, a function of certain design criteria imposed by the marketplace. A standard blade length of twenty-five feet has evolved, and the weight of the tape blade plus the amount of spring or other retractions means required to retract the tape blade can add up to almost a pound of material. These two components are often contained in a two-piece housing, generally orthogonal in configuration, and screwed together by at lease four screws equally spaced about the housing perimeter. This housing and other components provide the necessary rigidity to enable the tape to meet various use requirements, including passing certain "drop" tests. Since the housing was sized to accommodate the type, spring, locking elements, and the other parts, and to withstand the tests, in addition to being heavy, it is relatively large.
Two popular types of locks currently used include variations of a "knuckle joint" lock that works on the same principle as the knuckle in the finger, which is longer when straight than it is when it is bent. In addition, the joints in the fingers can flex slightly beyond straight (over-center), so that an increased compression force on the finger when the finger is "locked" (fully extended) causes the finger to stiffen in the straight position rather than to flex.
If one tries to incorporate either version of a knuckle joint lock into the tape case of the present invention, several problems are encountered. One problem is due to the fact that the overall size of the tape measure housing is determined, in part, by the size of the base which three inches is a convenient size for the hand of most users and it also avoids requiring the user to add fractional numbers if an inside dimension is measured. Another housing design problem results from the need for positioning the button for the tape locking mechanism in the upper forward quadrant of the tape case. This position is required to give the user easy access to the button with an index finger.
To use a "knuckle" type tape locking mechanism, there should be a path which is free of encumbrances or protuberances from the point at which the tape is locked in the bottom of the housing to the point in the upper forward quadrant of the tape case at which the button or other actuating mechanism pivots.
Tape locking systems, in general, employ a number of distinct elements, be they "knuckle" type locking systems or other type locking systems. Such distinct elements are the piece which attaches to the locking button and the locking shoe. These distinct elements present certain problems in the manufacture and long-term use of tape measures. Such distinct elements also make the tape locking systems quite sensitive to slight variations in the lengths of the two pieces which form the tape measure lock. If the length of either one of the pieces changes, due to variations in the molding process, wear on the bottom of the locking shoe, or other factors, the lock will quickly cease to be effective. This problem is aggravated by the placement of the lock button more distant from the locking point than is the case with prior tape measures. Thus, where the locking means are longer than normal, the possibility is increased that a small variation in the length of the locking shoe will affect the locking system enough to render it useless or reduce its effectiveness.
Similar problems are associated with the length of travel for the locking mechanism. Vertical travel is defined herein as the distance between the end of the locking shoe in the locked and unlocked positions. Inadequate vertical travel of the locking shoe can adversely affect retraction of the tape as the tape motor may not be able to overcome the friction remaining on the tape blade left by inadequate retraction of the locking shoe. For example, as the blade is extended, the amount of coiled tape blade left on the tape reel decreases. This changes the angle at which the tape blade reel approaches the mouth of the tape measure and consequently causes the locking shoe to rub on the tape blade. This rubbing is caused by the locking shoe not being moved a sufficient vertical distance from the tape measure to avoid interference with the tape. Rubbing slows the retraction of the tape. Rubbing necessitates a longer, stronger spring motor to overcome the frictional forces applied to the tape blade by the locking shoe. Hence, the absence of sufficient vertical travel may require the tape measure to be heavier and larger.
In some instances, the travel of the locking shoe is not strictly vertical or perpendicular to the path of tape travel; specifically, there is often a horizontal or parallel component to the locking shoe's motion at the end of its travel. This horizontal component tends to change the horizontal position of the blade, and hence changes the resultant measurement which defeats the essential purpose of having a tape measure.
Other problems with prior art tape measure locking systems are associated with the location and operation of the actuating mechanism. A horizontal push--toggle lock mechanism placed on the forward wall of the tape (just above the mouth where the tape blade exists the case) requires that the user either grip the tape extremely tightly or put one or two fingers behind the rear wall of the tape to prevent the tape from being pushed out of the hand. Placing one or two fingers behind the rear wall of the tape is awkward and uncomfortable. When the fingers are in this position, one locks or unlocks the tape by pressing the thumb toward the little finger. This is an awkward motion, and one in which the hand cannot exert much strength.
Accordingly, a need remains in the art, to provide a locking mechanism, which accurately locks a tape in position and is easily and comfortably used.